Combined current and voltage launcher for microwave cavity utilizing bicuneate plate



Oct. 5, 1965 J. F1 EHLERS COMBINED CURRENT AND VOLTAGE LAUNCHER FOR MICROWAVE CAVITY UTILIZ'L'NG BICUNEATE PLATE "Filed Jan. 14, 1965 INVENTOR. JHA/ United States Patent Office Patented ct. 5, i965 CM'BEINED CURRENT AND VOLTAGE LAUNCH- ER FR MICROWAVE CAVITY UTILIZING BI- CUNEATE PLATE .lohn F. Ehiers, Wichita, Kans., assigner to The Boeing Company, Seattle, Wash., a corporation of Delaware Filed `lan. 14, 1963, Ser. No. 251,254 Claims. (Cl. S33-21) This invention relates to a new and improved coaxial line-to-waveguide microwave `energy transducer or mode permuter, and is especially useful in applications requiring matched or stable impedance characteristics over wide bandwidths in a degree not readily achieved in previously available permuter devices. The invention is herein illustratively described in its presently preferred form; however, it will be recognized that certain modifications and changes therein with respect to details may be made without departing from the underlying essentials involved.

While the invention is especially suited for radar or communication cavity-backed slot or horn antennas requiring a coaxial-to-waveguide transducer, it has a wide variety of other applications as well including, but not limited to, laboratory equipment and electronic countermeasures systems. As, of course, in most all transducer applications, efficiency is essential.

Other objectives includes adequately high power handling capability, lightness lof weight, simplicity, ease of manufacture and assembly, rigidity against distortions due to vibrations and the like which could impair operating characteristics, adaptability of the configuration to successful operation in different frequency bands merely by scaling up or scaling down the dimensional specifications, and related objectives.

Certain prior proposals for coaxial-to-waveguide trans* ducers, less satisfactory than the present invention for wide bandwidth impedance matching characteristics, are described in Very High Frequency Techniques, Radio Research Laboratory Staff, Harvard University, McGraw- Hill, New York, 1947, page 717 et seq. That and other literature indicate the history of efforts to devise satisfactory mode permuters for the function described and something of the theoretical considerations involved therein.

As herein disclosed the present invention features a substantially flat plate-like conductor of quadrangle form which is symmetrical about the common .bisector of the corner angles at opposite ends -of the conductor. One such end corner is obtuse and its apex point serves as the terminus of an energizing coaxial transmission line conductor. The opposite corner angle is acute. The plate is mounted in transversely directed position Within an internally conductive elongated waveguide cavity of rectangular cross section, and in spaced parallel relation to the bottom wall -of the cavity. The transmission line conductor enters through a side wall aperture in the cavity to join the plate. The cavity may itself serve as a radiator or may be connected to a waveguide or horn device, as desired.

As an important feature, a conductive rod extends between the underside of the plate and the cavitys bottom wall. It extends substantially parallel to the conducting surface containing the side wall aperture and spaced therefrom along the plates corner bisector. The rods length is approximately lone-quarter of the guide wavelength at the highest operating frequency used. Still another important feature is the provision of a dielectric block interposed between the plate and bottom wall of the cavity. This block extends generally parallel to the rod and is positioned so that its opposite side portions lie symmetrically about the plates end corner angle bisector. The block is located intermediate the ends of the plate.

The plate may be viewed as a composite yof short and long wedges (equilateral triangles) having a common base. The conductive rod is located intermediate the common base and the apex of the short wedge and the plate is located intermediate the common base and the apex of the long wedge.

These and other specific features, objects and advantages of the invention will become more fully evident from the following description thereof and by reference to the accompanying drawings.

FIGURE l is a top view of the coaxial-to-Waveguide transducer located in a waveguide cavity.

FIGURE 2 is a sectional side view of the coaxial-towaveguide transducer taken von line 2-2 of FIGURE l.

FIGURE 3 is a perspective View of the coaxial-to-waveguide transducer located in a waveguide cavity.

FIGURE 4 is an electrical circuit equivalent of the mode permuter contained in a waveguide cavity.

As illustrated in the drawings, the xenobicuneate mode permuter comprises a voltage coupling quadrangular conductor 10 which in a practical case comprises a unitary metal mode permuter plate 10. 'This plate has an acute angle end corner M and an opposed obtuse tangle corner N both bisected by the longitudinal axis A about which the plate l0 is symmetrical. The side corners O and P of the plate 10 form equal angles normally just less than a right angle. The plate 10 is mounted in an opend-sided cupshaped conductive waveguide section or cavity 15 having closed ends 15a and 15b. The plate 10 is mounted parallel to the cavity bottoms 15C at a location intermediate the top and bottom of the cavity 15. A longitudinal hole 12 drilled in the obtuse angle end corner N of the plate 10 accepts the coaxial transmission line center conductor 11. For purposes of electrical continuity the parts 10 and 11 are joined in the hole 12 by threading, by a press fit, by silver soldering7 or by any other suitable method. The transmission line conductor 11 enters the cavity l5 through a side Wall 15d aperture having an insulating support bushing 13 therein.

A cylindrical conductive rod 14 is threaded at its upper or outer end 14a into a tapped hole in the lower or inner side of the mode permuter plate 10 at an axial location close to the corner N. The diameter of this rod 14 is stepped up at its lower end 14h and is drilled and tapped to receive a mounting screw passed through the bottom wall 15e of waveguide cavity 15. A dielectric platesupporting block 16 extends between the inner side of plate 10 and the bottom wall 150 of the waveguide cavity 15. In order that the dielectric block 16, which serves a supporting function only, will not interfere with the electrical characteristics of the mode permuter 10 its volume should be kept at a minimum. To this end, the block 16 in its preferred form comprises two parallel posts disposed on opposite sides of the plates longitudinal axis A and interconnected by a web 16e. Two sets of securing screws 16a and 16h pass through holes in the plate 10 and cavity bottom wall 15o respectively and thread into the ends of the post portions of the block 16.

The location of block 16 along the plate 10 is not critical although it is preferably situated approximately midway between the tip of the corner M and the rod 14, a location at which it imparts a high degree of mechanical stability to the plate 10.

The improved mode permuter was designed initially for a frequency band (P-band) from 500 megacycles to 1000 megacycles. It should be understood, however, that the design requirements are essentially the same for operation in other frequency bands (such as L-band or S-band), and that the physical scale of dimensions of the elements will be decreased or increased in accordance with the operating wave lengths chosen.

Typically, the mode permuter plate is cut from a flat metal sheet such as .125 inch stock. The precise thickness of the sheet is not critical, although it should be thin in relation to the major physical dimensions of the completed element 10. As described, the outer edge 13a of the plate 10 is preferably a quadrangle symmetrical about axis A, and may be defined by two wedges of different apex angles at the corners M and N directed oppositely from a common base extending between the corners O and P. The corners O, P and M are rounded at 1817, 13C and 18d, respectively. The contour followed by this rounding is not critical, that is, it may comprise a portion of a circle, of an ellipse, or of a parabola. The effect is primarily one of increasing the power handling capability of the mode permuter.

The dimensions primarily determining the performance of the mode permuter are the combined and relative lengths of the two component wedge shapes, and the width of their common face. It has been determined that the total length of the plate 10 should be approximately 80% of the shorter width of the coaxial cavity between the side walls 15a and 15b in which it is placed (i.e., measured in the direction parallel to axis A). The width of the plate 10 (i.e., between corners O and P) should be approximately equal to its length. The length of the wedge shape NOP which is connected to the transmission line conductor should be approximately one-fourth the combined length of the two wedges NOP and MOP.

A lumped parameter electrical circuit equivalent of the permuter is depicted in FIGURE 4. The conducting wall surfaces of the side walls 15a, lSb, iSd, 15e and bottom wall 15C of waveguide cavity l5 to which the conducting rod 14 is connected represents a susceptance in parallel with a load admittance obtained from the open side of the waveguide cavity. The effective length of the permuter as a dipole taken along axis A is designated (I). The ratio of this length to the waveguide width (b) measured along axis A determines the turns ratio (Z/b) of an equivalent autotransformer and hence the magnitude level of the admittance presented to the coaxial conductor 1l. This dipole length is equivalent to the length of a thin probe having uniform current distribution. The value of this effective dipole length of the xenobicuneate mode permuter can be found approximately by assuming the TElY mode in the waveguide l5, finding the total current on the mode permuter as a function of its length, and computing the average current-length product. The dipole length is then found by dividing the current-length product by the maximum current amplitude. Inasmuch as the width of the plate lt? affects the current distribution, and hence the equivalent dipole length of the mode permuter, there is no unique solution for either the lengthwidth dimensions of the plate l0; that is there are several combinations of length and width which yield the same equivalent dipole length.

With further reference to FIGURE 4, for purposes of maximum coupling between the permuter and transmission line under the required broad-band operating conditions, the susceptance presented at the input of the equivalent auto-transformer must be resonant with the selfreactance of the mode permuter over a wide frequency range. This self reactance of the mode permuter can be determined with approximate accuracy by considering the permuter as an open-end transmission line. The resulting characteristic impedance of the longer wedge shape is expressed as where l1 is the length of the longer wedge ab is the width of this wedge near the apex thereof at is the maximum width of the wedge The characteristic impedance of the shorter wedge shape is expressed as Z0=6O ln at where l2 is the length of the shorter wedge at is the maximum Width of this wedge Again it is to be noted that both the length and width of the wedge directly affect the permuter impedance. Moreover, for optimum operation there is a most suitable combination of these dimensions in order to satisfy both the dipole length and self-reactance conditions. The aforementioned approximate relationship of the dimensions are considered optimum for a match to a 50 ohm transmission line.

With still further reference to FIGURE 4, it has been established that the self-reactance of the mode permuter is modified by the reactance of the conductive rod 14 in a manner which further extends the bandwidth of the mode permuter. The reactance of this rod can be determined approximately by considering it as a short circuited transmission line Whose characteristic impedance is given by where D is the lateral distance between the axis of the rod and the adjacent (apertured) side wall of the waveguide cavity 15 C is the diameter of the rod The xenobicuneate mode permuter is highly suitable as a coaxial-to-waveguide transducer, especially in the microwave regions such as the P and L-bands. Because of its special geometrical form in association with the conducting rod 14, al1 impedance matching is accomplished within the mode permuter itself, eliminating the necessity for exterior tuning apparatus. It is efficient and, as will be seen, is of relatively simple construction.

By the proper choice of a structural dielectric material for the dielectric supporting block 16 the mode permuter is extremely rigid, so that its operating characteristics are not influenced appreciably by any vibrational effects which may occur in the surrounding structure.

These and other aspects of the invention will be recognized by those skilled in the art on the basis of the foregoing disclosure of the presently preferred embodiment thereof.

I claim as my invention:

tl. A broad-band coaxial-to-Waveguide microwave coupling device comprising, in combination with a coaxial line center conductor and a waveguide section having an apertured side wall through which the center conductor enters the section along a direction generally parallel to an adjoining wall, a substantially flat platelike conductor of generally quadrangular form having two opposite side corners and having opposite end corners respectively dening obtuse and acute angles with a common bisector, said plate-like conductor having opposite side portions symmetrical about said bisector as an axis of symmetry, said plate-like conductor being connected at said obtuse angle corner thereof to the center conductor in substantial alignment of said axis therewith, the width of said plate-like conductor transverse to said axis approximating the length along said axis, and means extending generally transverse to the general plane of said plate-like conductor connecting the same to the adjoining Wall, and said means including a conductive rod extending generally parallel to the apertured wall at a location near said obtuse angle corner of said plate-like conductor.

2. The device defined in claim l, wherein the length of said plate-like conductor along said axis approximates 80% of the width of the waveguide section measured in the same direction, and wherein said obtuse angle corner connected to the center conductor is positioned adjacent to the apertured wall of the waveguide section.

3. The device defined in claim 1, wherein said two opposite side corners and said acute angle end corner of said plate-like conductor are rounded on a substantial radius.

4. The device defined in claim 1, wherein said two opposite side corners of said plate-like conductor are transversely aligned at a location along said axis approximately one-fourth the distance from said obtuse angle corner to said acute angle corner.

5. The device defined in claim 4, and a dielectric block supportingly interconnecting said plate-like conductor and the adjoining wall of the waveguide section at a location intermediate the ends of said plate-like conductor.

6. The device defined in claim 1, wherein said rod is in direct electrical contact with the adjoining wall.

7. The device defined in claim 6, wherein the length of said rod is approximtaely one-quarter of a waveguide wavelength at the highest frequency of the broad-band.

8. A broad-band coaxial-to-waveguide microwave coupling device for connection to a coaxial line center conductor and a waveguide section having an apertured side wall through which the center conductor enters the waveguide :section along a direction generally parallel to an adjoining wall and comprising, a substantially flat plate-,like conductor of generally quadrangular form having opposite end corners respectively defining obtuse and acute angles with a common bisector, said plate-like conductor having opposite side portions symmetrical about said bisector as an axis symmetry, said plate-like conductor being adapted for connection at said obtuse angle corner to the line center conductor in substantial alignment with said axis, the width of said plate-like conductor transverse to said axis approximating the length thereof along said axis, and means extending generally transverse to the general plane of said plate-like conductor for connecting the same to the adjoining Wall, and said means including a conductive rod joined to said plate-like conductor at a location near said obtuse angle corner thereof.

9. A broad-band coupling device comprising, a coaxial line fitting including a center conductor, a conductive enclosure comprising `a first wall apertured to pass said center conductor, an adjoining wall transverse to said first wall and :spaced transversely from said center conductor, a conductive plate of quadrangular form having one corner conductively joined to said center conductor coextensive with said axis of said center conductor, said axis bisecting the angle of said one corner and an opposite corner of said quadrangle, said quadrangle being substantially symmetrical about said axis and disposed substantially parallel to said adjoining wall, and a conductive post bridging transversely between said adjoining wall and said conductive plate at a location on said axis adjacent said one corner.

10. The device defined in claim 9, and a plate-supporting dielectric block bridging transversely between said adjoining wall and said conductive plate at a location intermediate said conductive post and said opposite corner of said quadrangle.

11. A broad-band coupling device comprising, conductive waveguide means, coaxial conductor means extending into and dielectrically insulated from said conductive waveguide means, quadrangularly--shaped conductive probe means having one corner conductively connected to said coaxial conductor means and having an opposite corner substantially aligned with the axis of said coaxial conductor means and said one corner, conductive means electrically connecting said conductive probe means to said conductive waveguide means, and dielectric means supporting said conductive probe means within said conductive waveguide means.

12. A broad-band coupling device as set forth in claim 11, wherein said conductive waveguide means comprises first, second, third, fourth planar side walls of equal height, said first and second planar side walls being opposed and parallel and of equal dimensions, said third and fourth planar side walls being opposed and parallel and of equal dimensions and perpendicular to said first and second planar side walls, a planar bottom wall perpendicular to said planar side walls and contiguous therewith to form a rectangular cup-shaped conductive waveguide box, said coaxial conductor extending through said first planar side wall, said rst and second planar side walls being longer than said third and fourth planar side walls, said dielectric means and said conductive means being connected to said bottom wall.

13. A broad-band coupling device as set forth in claim 12, wherein said conductive probe means has first, second, third and fourth rectilinear sides, said first and second sides intersecting at an obtuse angle and defining a first corner conductively connected to said coaxial conductor means, said third and fourth sides disposed at an acute angle with the other, said first and fourth sides disposed at an obtuse angle with the other, and said second and third sides disposed `at an obtuse angle with the other.

14. A broad-band coupling device as set forth in claim 13, wherein said conductive means are connected to said conductive probe means between lsaid first corner and a second corner defined by said third and fourth sides and closely adjacent to said first corner, said first corner being spaced from said planar side walls, and said conductive probe means being disposed parallel to said bottom wall.

15. A broad-band coupling device as set forth in claim 14, wherein said dielectric means comprises: two parallel posts with an interconnecting web disposed perpendicular to said conductive probe means and parallel to said first planar side wall.

References Cited by the Examiner UNITED STATES PATENTS 2,431,124 11/47 Kees et al 343--789 2,973,517 2/61 Watts 343-908 HERMAN SAALBACH, Primary Examiner, 

1. A BROAD-BAND COAXIAL-TO-WAVEGUIDE MICROWAVE COUPLING DEVICE COMPRISING, IN COMBINATION WITH A COAXIAL LINE CENTER CONDUCTOR AND A WAVEGUIDE SECTION HAVING AN APERTURED SIDE WALL THROUGH WHICH THE CENTER CONDUCTOR ENTERS THE SECTION ALONG A DIRECTION GENERALLY PARALLEL TO AN ADJOINING WALL, A SUBSTANTIALLY FLAT PLATELIKE CONDUCTOR OF GENERALLY QUADRANGULAR FORM HAVING TWO OPPOSITE SIDE CORNERS AND HAVING OPPOSITE END CORNERS RESPECTIVELY DEFINING OBTUSE AND ACUTE ANGLES WITH A COMMON BISECTOR, SAID PLATE-LIKE CONDUCTOR HAVING OPPOSITE SIDE PORTIONS SYMMETRICAL ABOUT SAID BISECTOR AS AN AXIS OF SYMMETRY, SAID PLATE-LIKE CONDUCTOR BEING CONNECTED AT SAID OBTUSE ANGLE CORNER THEREOF TO THE CENTER CONDUCTOR IN SUBSTANTIAL ALIGNMENT OF SAID AXIS THEREWITH, THE WIDTH OF SAID PLATE-LIKE CONDUCTOR TRANSVERSE TO SAID AXIS APPROXIMATING THE LENGTH ALONG SAID AXIS, AND MEANS EXTENDING GENERALLY TRANSVERSE TO THE GENERAL PLANE OF SAID PLATE-LIKE CONDUCTOR CONNECTING THE SAME TO THE ADJOINING WALL, AND SAID MEANS INCLUDING A CONDUCTIVE ROD EXTENDING GENERALLY PARALLEL TO THE APERTURED WALL AT A LOCATION NEAR SAID OBTUSE ANGLE CORNER OF SAID PLATE-LIKE CONDUCTOR. 